The air receiver tank is often overlooked when it comes to sizing compressed air systems. Sized properly it can make a compressor run more efficiently.
Primary Air Reciever Tank
Primary air receivers in a compressed air system are located after the compressor and the aftercooler but before any filtration or drying equipment.
The rule of thumb to use for sizing air receivers is:
compressor capacity (CFM) = air receiver size (gallons)
The actual formula is:
The primary air receiver’s responsibility includes the following:
Contaminant Removal - Air receivers provide an additional separation point for contaminants and liquid within the compressed air.
Air generally enters from the compressor and leaves to go to the system piping at high speed. With a properly sized air receiver, the compressed air is allowed to slow down. The residence time of air within the receiver tank depends on many system variables. Solid and liquid particles that were entrained within the compressed air are allowed to settle-out as the air slows down in the air receiver tank.
Liquids not drained from the tank can cause rust. A drain should be installed at the lowest point of the tank to remove contaminants. Unless a manual drain will be opened on a regular basis, an automatic-timer drain should be used. In order to provide necessary drainage, a horizontal tank can be raised slightly on one end to promote flow of contaminants to the drain end of the receiver tank.
Pressure Spike Dampening - Positive displacement compressors create surges in the compressed air supply (some more than others).
The volume contained within a receiver tank helps to dampen the surges from the compressor leading to a more consistent pressure supply.
The receiver tank also helps to protect the compressor from fluctuations in air requirements. An air tank allows the compressor to build up a store of compressed air each time it turns on. With the store of air available to downstream processes, the compressor can sit idle for longer periods of time and will operate more efficiently.
Secondary Air Reciever Tank
A secondary air receiver tank has a slightly different purpose than the primary air tank. It is installed near a piece of equipment that operates intermittently.
The advantage of installing secondary tanks at locations where higher flow rates are required on an intermittent basis is that if an intermittent process can be averaged out (which an air tank will do), the instantaneous requirement of the compressed air system is reduced.
Air Receiver Code Requirements
Many regions have a code requirement for the construction of air receiver tanks.
The American Society of Mechanical Engineers (ASME) has established a code for air receiver tanks and many companies rate their tanks to that code.
Safety Relief Valves
All tanks that contain compressed air should have a safety relief valve.
A rule of thumb is to set the relief valve to 10% than the highest system pressure requirement. But the relief valve should never be set higher than the pressure rating of the tank it is connected to.
All tanks should have a condensate drain to remove liquid from the tank.
If the manual drain cannot be opened on a regular basis, an automatic-timer drain should be used.
The tank should have a large pressure gauge mounted to it. By using a pressure gauge rated to double the operating pressure, the normal needle location will be pointing straight up. A gauge snubber should also be used to protect the gauge from pressure spikes.
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